The invention relates to the field of RF power detectors and controllers, and in particular to RF power detectors and controllers used in mobile handset terminals for wireless applications.
The demand for greater bandwidth for mobile systems in a highly competitive landscape has forced network providers to look for low cost ways to expand existing networks. One such system that is being used is Enhanced Data Rate for Global Evolution (EDGE). The EDGE system enables the providers to expand the existing Global System for Mobile Communications (GSM) system, the standard for 70% of the worldwide market, to offer bandwidths of up to 300 kbps. This bandwidth and capacity extension can be accomplished with a relatively small upgrade to the existing infrastructure network. The system, however, does present some technical challenges. One of these is in the transmitter section in the handset terminals.
The EDGE system uses a new modulation scheme to add the additional data capacity. The GSM system uses a Gaussian Minimum Shift Keying (GMSK) algorithm to encode the data in such a way that the transmitted signal has a constant envelope. The EDGE system, on the other hand, uses an 8-phase key shifting (PKS) algorithm that has an envelope with a 3.3 dB peak-to-average ratio; and a 17 dB overall variation in the amplitude. This amplitude variation requires that the transmitter amplify the signal with linearity. The most difficult section of the transmitter to make linear is the Power Amplifier (PA). The PA must produce the required amount of power to the antenna, with good DC to RF conversion efficiency to prolong battery life. This additional stipulation for linearity makes meeting the power and efficiency requirements much more difficult. Additionally, the transmitter and PA must also operate in GSM mode. Very little performance degradation can be tolerated in this mode since it will be used for the majority of voice traffic. Unfortunately, a PA designed to operate in a linear mode for EDGE will not be able to obtain the same peak efficiency in GSM mode.
Another problem arises with the detection and control of the RF output power. The existing GSM system has very rigid requirements for power ramping. Because the system is Time Division Multiple Access (TDMA), the transmitted energy falls into specific slots, or bursts, in time. Thus, a power control and detection scheme is used to control the power ramp and power level. A feedback control system is commonly used for this purpose. To meet the rise time requirements, a loop bandwidth on the order of several hundred kHz is needed. The EDGE system has similar ramping requirements. The AM modulation contained in the signal, however, is also on the order of about 280 kHz. This means that the feedback loop used for GSM would eliminate the amplitude modulation and distort the signal.
There is a need, therefore, for a system and method to more efficiently and economically providing an EDGE system.